I. Field of the Invention
The present invention relates to the use of refractory metals or metal silicides as high conductivity metallization layers of MOSFETs. Specifically, the present invention is related to an integrated circuit having polycrystalline silicon deposited on high-conductivity metal silicide to form an interconnect layer for interconnecting selected gates and substrate areas of a MOSFET.
II. Description of the Prior Art
The increasing complexity of integrated circuit designs has required the development of multilayer interconnect processes. Typical two-layer interconnect processes have either two metal layers or one polycrystalline layer and one metal layer. The latter process is widely used in the manufacture of LSI MOS circuits. However, especially with regards to VLSI circuits, the relatively high resistivity of polycrystalline silicon (as compared to aluminum) substantially limits circuit performance. The use of refractory metals and refractory metal silicides for interconnect layers has been proposed to replace polycrystalline silicon in MOS and bipolar IC's in order to take advantage of the relatively low resistivity of these materials and their high stability at the high temperatures associated with many IC processing steps. A number of authors have recently reviewed the use of refractory metals or silicides as interconnect materials. For example, see T. Mochizuki et al., Japanese Journal of Applied Physics, 17, 37 (1978) and B. Crowder et al., IEEE Journal of Solid State Circuits, Vol. SC-14, No. 2, Apr. 1979, p291-293. "1 .mu.m MOSFET VLSI Technology: . . . "
However, the use of silicides in VLSI Technology causes a number of problems. Specifically, it is difficult to provide a good ohmic contact between a silicide interconnect layer and a silicon substrate. Conventionally, the substrate is doped through the polycrystalline to establish a degree of continuity in doping between the polycrystalline layer and the substrate which improves the ohmic contact. However, silicides are diffusion barriers, which makes it impossible to form a good ohmic contact between a silicide interconnect layer and a substrate by the conventional method of doping the interconnect layer and driving the doping through to the contact area of the substrate. Secondly, silicide does not mechanically bond well to either polycrystalline silicon or to single crystal silicon. This is especially true when the silicide is deposited at a low temperature. Thirdly, silicides, especially when deposited by sputtering, do not always provide good step coverage over the vertical features of the integrated circuit. That is, sputtered silicides may be characterized by electrical discontinuities when they are deposited over vertical steps associated with integrated circuit surfaces. This is especially critical when the silicide is used by itself rather than in combination with a polycrystalline silicon. Finally, it is necessary to avoid any possible reaction between the silicide and the contacting layers or chemicals which will come in contact with the silicide during the manufacturing process.